The present invention relates to radio communication systems. More particularly, and not by way of limitation, the present invention is directed to an apparatus and method for mitigating interference between radio users operating in the same frequency band.
Radio operations have recently been allowed in the frequency range 3.1-10.6 GHz. Instead of a fixed allocation of radio bands, the new radio transmissions will overlap with existing services. This is accomplished by the use of so-called Ultra Wideband (UWB) modulation techniques. A signal is denoted UWB when it either has a bandwidth of at least 500 MHz, or has a fractional bandwidth larger than 0.2. The transmit power is spread over a large frequency range, resulting in a low power spectral density (PSD) which is measured in dBm/MHz. The emission levels established by the FCC are rather low, resulting in a severe limitation of the range of UWB radio systems. UWB systems are therefore typically used in short-range systems providing Wireless Personal Area Networking (WPAN).
UWB emissions may interfere with the communication of other, narrower-band, systems operating in the 3.1-10.6 GHz band. However, these so-called victim systems have much narrower bandwidths, and therefore will experience the UWB emission as an increase in the noise floor. Systems that will be impacted by the UWB emission include Fixed Services (Wireless Local Loop) operating around 3.4 GHz, Wireless Local Area network (WLAN) systems operating around 5 GHz, and envisioned cellular systems (beyond IMT-2000) that may operate at frequencies between 3 and 6 GHz.
There are several occasions in which the increase in noise floor in the victim receivers in not acceptable. When a mobile terminal is at a cell border or finds itself under heavy fading conditions, it may operate at the limit of its sensitivity. Any increase of the noise floor, caused by a UWB system, will result in an unacceptable performance degradation. Thus, in essence, the UWB emission reduces the range of the victim system. This range reduction can only be compensated for by using more base stations, which is not a desirable solution.
Many UWB developers are, therefore, working on interference mitigation techniques. Proposed solutions include having the UWB devices scan the frequency spectrum of interest, and when they detect an existing system (for example a Worldwide Interoperability for Microwave Access (WiMAX) system at 3.41 GHz), they avoid that part of the spectrum. This technique is called Detect-And-Avoid (DAA). Such avoidance, however, is not a trivial task. Since the UWB transmission is very wide (0.5-1 GHz), a notch must be created within the transmit spectrum. In addition, the notch must be deep enough (for example 30 dB or more) in order to limit the UWB emission at the frequency to be avoided. Furthermore, the location of the notch must be variable since the victim carrier frequency is not fixed.
The current UWB technologies can be divided into three classes:
a) Orthogonal Frequency Division Multiplexing (OFDM) based: this is an up-scaled OFDM technique with 128 sub-carriers and a carrier spacing of 4.125 MHz, rendering a total bandwidth of 528 MHz.
b) Direct-Sequence Spread Spectrum (DSSS) based: this technique uses chip rates on the order of 1-2 Gchips/s with variable spreading factors.
c) Pulse based: this technique uses very short pulses with a pulse duration that is a fraction of a nanosecond. Most pulse-based techniques use Pulse Position Modulation (PPM) to carry the information.
With the OFDM technique, variable notches with a depth of approximately 20 dB can be obtained by inactivation of specific sub-carriers. With more advanced techniques (for example, using a dummy sub-carrier to compensate), at most 30 dB can be obtained. In some cases, however, notches deeper than 30 dB may be required. For the DSSS and pulse-based techniques, variable notching is an even larger problem, making it much more difficult to obtain notch depths equivalent to those obtainable with OFDM. Use of the DSSS and pulse-based techniques is desirable, however, because in contrast to OFDM-based UWB, use of these techniques results in very low implementation costs.
A method and apparatus for adapting multi-band UWB signaling to interference sources is described in U.S. Patent Application Publication No. US 2004/0048574 (Walker et al.). However, Walker et al. do not disclose or suggest any methodology utilizing notches in a UWB spectrum to mitigate interference.
Thus, what is needed in the art is an apparatus and method for interference mitigation that overcomes the deficiencies of conventional systems and methods, and can be used with OFDM, DSSS, and pulse-based UWB techniques. The present invention provides such an apparatus and method.